Single crystalline silicon in the form of a thin sheet is desirable for several important reasons. It achieves materials savings in silicon utilization and provides product use possibilities in photovoltaic (solar cell) installations. It also offers a very low cost potential of making semiconductor ribbon by continuous process means for subsequent device fabrication.
Several investigators have explored various methods of growing single crystalline silicon sheet. Some of the known art for the purpose is revealed, amongst other sources, in: an article by A. V. Stepanov in Soviet Physics-Technical Physics, 49, 339 (1959); an article by J. C. Swartz, T. Surek and B. Chalmes in J. Electronic Materials, 4, 255 (1975); a report dated Dec. 15, 1975 prepared by G. H. Schwuttke et al entitled "SILICON RIBBON GROWTH BY A CAPILLARY ACTION SHAPING TECHNIQUE" identified as Quarterly Technical Progress Report No. 2 under JPL Contract No.: 954144 (a Subcontract under NASA Contract NAS70100 -- Task Order No. RD-152; and a presentation of J. A. Zoutendyk at the "International Solar Energy Society Conference `Sharing The Sun`" made Aug. 16 - 20, 1976 at Winnepeg, Manitoba, Canada and entitled "DEVELOPMENT OF LOW-COST SILICON CRYSTAL GROWTH TECHNIQUES FOR TERRESTRIAL PHOTOVOLTAIC SOLAR ENERGY CONVERSION". U.S. Pat. Nos. 3,393,054; 3,591,348; and 3,617,223 also pertain to this technology, as does the copending U.S. Patent Application entitled "APPARATUS FOR THE PRODUCTION OF RIBBON SHAPED CRYSTALS", Ser. No. 680,072, Filed Apr. 26, 1976.
Three newer processes used for single crystalline silicon sheet growth are known as: (i) the "Stepanov Technique"; (ii) the "Inverted Stepanov Technique"; and (iii) "Edge-Defined Film-Fed Growth" (i.e., "EFG"); plus various modifications thereof. The more conventional Czochralski silicon crystal growth method is also used. One of the essential differences between the first three processes lies in the contact angle (.phi.) between the molten silicon and the die material. For EFG, .phi. should be much smaller than 90.degree.. On the other hand, .phi. should be larger than 90.degree. in the Stepanov Technique, while it can be more or less than 90.degree. in the Inverted Stepanov Technique.
Molten and near molten (e.g., solidifying from the melt) silicon is extremely reactive. It tends to attack or corrode and/or become undesirably contaminated by practically all of the die and crucible materials that have been employed for growing single crystalline silicon sheet. This, obviously, is very undesirable.
A copending U.S. Patent Application entitled "Apparatus Means for Growing Single Crystalline Silicon Sheets", Ser. No. 732,865, filed Oct. 15, 1976, discloses that coating dies and crucibles with a layer of silicon nitride does protect the die and crucible materials from attack by and corrosion of molten silicon.